Numerous ferrous alloys (e.g., high strength steels) and non-ferrous alloys have been developed for use in heavy construction and machinery. Although these alloys provide a good combination of strength and toughness, they typically do not show adequate resistance to wear, erosion, and corrosion. Thus, they are not well-suited for use in applications in which the surfaces of these alloys are subjected to aggressive environment or abrasion. One approach to remedy this problem is to use a hard-facing material deposited onto the surface of an underlying structure/substrate to act as a protective layer. The underlying structure (e.g., steel substrate) provides the strength and structural integrity needed for the layer-substrate structure, and the hard-facing alloy protects the substrate against wear and abrasion in adverse environments. The hard-facing material also can protect the substrate against corrosion as well.
A wide-variety of hard-facing materials are known, including, for example, ceramic-containing compositions such as tungsten carbide/cobalt and purely metallic compositions. One problem encountered with most hard-facing material is that when applied by thermal spraying, the hard-facing deposit often contains porosity and has through-cracks that extend perpendicularly to the thickness direction of the coating. The porosity permits corrosive media to penetrate through the coating to reach the substrate and damage it by chemical corrosion or stress corrosion. The through-cracks can also lead to fracturing and spalling of the wear-resistant coating, thereby resulting in the abrasive or corrosive media reaching the underlying substrate and rapidly wearing out the underlying substrate.
Another class of metallic hard-facing materials is the frictionally transforming amorphous alloys generally disclosed in U.S. Pat. No. 4,725,512. These ferrous materials can be deposited upon the surface of a substrate as a hard-facing layer in their non-amorphous state by techniques such as thermal spraying. When the hard-facing layer is subjected to wearing forces, such as abrasive wear, the deposited material can metamorphically transform to a hard, wear-resistant amorphous state. Another class of alloys is titanium-containing ferrous hard-facing material, which are disclosed in U.S. Pat. No. 5,695,825. Although these hard-facing alloys are suitable for certain applications and used extensively as coatings in drill-pipes, improvements are still desired, especially for the applications wherein the adverse environment degrades the abrasion, erosion and corrosion characteristics of the alloys.
Thus, there is a need to overcome the aforedescribed challenges in a manner that does not adversely affect the basic operability of these materials for hard-facing applications.